Sixteen men stole an undisclosed number of diamonds from a mine near Prieska on Monday morning, Northern Cape police said.
"This morning at about 03:40, about 16 men stormed into a local mine near Prieska," said Lt Olebogeng Tawana.
Mine security guards and two employees were tied up and assaulted and the men demanded the safe keys. After they could not get access to the safe, they broke into a room used for sorting the diamonds.
The value of the stolen diamonds had not been established. One person was in custody for questioning, while the others were still at large.
"We're investigating a case of robbery," said Tawana.
He appealed to anyone with information about the robbery to contact police.